libphobos on ARM

[Johannes Pfau]

Am Mon, 11 Jun 2012 18:30:37 +0100
schrieb Iain Buclaw <ibuclaw at ubuntu.com>:
> 
> There are two things under my general consensus for this:
> 
> 1. would be to re-implement dfrontend/todt.c entirely, so that we
> produce GCC trees directly from the toDt routines, rather than the
> dmd's intermediate backend representation and later blindly converting
> to GCC after all information about the type size is finalised.
> 
> 2. would be to review the current implementation of how we record
> inheritance in classes and fix it up where possible to utilise the
> already existing macros in place to hold information about type
> inheritance and basetypes for the backend to better understand what
> information we are sending it.

I'm not very familiar with the GDC glue code yet, but I'm not sure if 1
would help in this case.

The problem is that ClassDeclaration::toSymbol() in d-decls.cc uses
ClassDeclaration::classinfo->type->toCtype() to generate it's Stree
field and therefore uses size = 76.
Then outdata (in d-objfile.cc) calls check_static_sym which returns
this Stree(size = 76). outdata the sets DECL_INITIAL(t) =
dt2tree(sym->Sdt), and this dt2tree result produces data with size =
108.

Now if we kept the initializer sym->Sdt as a tree instead of the dt*
list stuff, we could still have size mismatches. So while getting rid
of the intermediary dt_t representation sure has lots of other
benefits, I don't see how it could be useful in this case. We'd have to
somehow combine the dt_t creation (in this case it's actually not toDt,
it's ClassDeclaration::toObjFile which creates the dt_t list) and
ClassDeclaration::toSymbol() to really avoid this problem, but that's
probably a lot of work.


2 seems like it probably wouldn't be ABI compatible to dmd. I don't
care if we break ABI compatibility, just wanted to mention that.


I guess we can't just change the size of classinfo->type->toCtype() to
match the initializer size? We'd probably have to create a custom
type for every ClassDeclaration then? Something like
RECORD(classinfo->type->toCtype(); ubyte[x])?

----------------------------------------------------------------------
BTW: We should probably add something like
if(DECL_INITIAL(t) != NULL_TREE)
    gcc_assert(DECL_SIZE(t) >= TYPE_SIZE(TREE_TYPE(DECL_INITIAL(t))))

(or probably those even need to be equal, I'm not sure if the gcc
backend adds padding automatically)

to outdata?
Wouldn't this detect most of these bugs?